Benson Monarch

[pullshocks]

What volume setting on the amp during your test? I have not turned up past 2
The Ampbooks paraphase calculator indicates a pot setting 4k ohms would be balanced. How does that line up with what you saw with the signal generator?

I think it was between 2-4 during that test
I will set it to 4K and see what I get . That would be at 40% rotation and I was getting a good balance between low noise floor and good tone there

Have you played around any more with the trim pot? I tried setting mine by matching the voltage on my multimeter measured at the grids to the power tubes. They matched voltages at 3.9K. Sounds good, but I haven't had a chance to turn up much since the change. It seems a hair louder at this spot. It was a bit off from where I had set it by ear.

Mike

I think I have my PI balance pot hooked up backwards from TWilcox and mikeywoll. So fully counterclockwise is 0 ohm, fully counter clockwise is 11 k ohm (measured). You can see it on the front panel, with the resistances crudely inked in. EDIT Finally have an amp that goes to 11.

When you guys are talking about PI balance pot rotation I would appreciate you indicating what resistance you are talking about. Thanks

I find that as the resistance is increased, the noise increases. I measured the hum SPL on speaker axis directly at the grill cloth as described in an earlier post. With this method, I see a pretty linear increase of 1 dB per k ohm. No dips, nothing funny

With a guitar plugged in, the higher resistance, the louder it gets. It sounds good no matter what the setting, maybe fuller at the highest resistance settings, but that could be the classic psychoacoustic "louder sounds better" effect. The power of this amp limits me to volume settings no higher than 4, and in that range and with Kinman strat pickups, I am not getting any overdrive to speak of, so can't comment on the effect of PI balance setting on overdrive.

I changed the "voice" switch per mikeywoll's post. Much better.

I happened to see the schematic of the Fender 5e9 today, and it has a 12AX7 paraphase PI. The plate resistors and cathode resistor are 220k/1.5k compared to the Monarch's 100k/470 ohm. I plugged these numbers in the Ampbooks paraphase calculator. It gives a value of 3.45k for R2, equivalent to the resistance of the monarch PI balance pot. The value Fender used is more than double that. Food for thought.

Speaking of Ampbooks, found a couple other interesting reads that are relevant

https://www.ampbooks.com/mobile/amp-tec ... -follower/ It gives quite a bit of info about the Fender paraphase PI before going into the more elaborate Ampeg circuit.

https://www.ampbooks.com/mobile/amp-tec ... t-7145392/ It has to do with sonic effect of unbalanced signal to power amp.

[BobL]

Anyone still running AC heaters and gotten rid of the hum? Wondering if that's what I need to look at next... have the parts, need to figure out space.

[pullshocks]

Hrm... well, made the following changes to mine tonight:

Added a 4th node, and switched to 22/47/22/22 with 4.7K between nodes B and C and C and D.

Sent node D to the preamp.

Sent grounds for nodes C and D to the same ground as the preamp.

Swapped V2 pins 2/7.

Replaced the 1K resistor on the reverb return grid with 2.2K.

Swapped the A/B switch to the middle lug.

Seems to be humming just as much as it ever did. It's late, so haven't really had a chance to play it or mess with the reverb. Flipping the voicing switch introduces a pretty good pop going from A to B, and does seem to change the tone more. PI balance pot seems to have a different impact on the nature of the hum now... introduces a hiss vs changing the tone of the hum, but I can get rid of the hiss at certain settings. Hum seems here to stay.

Made so many changes, it ain't particularly pretty in there at this point, but pretty confident it is all wired as per my layout.

Do you have the DC heaters implemented? If still running AC heaters, did you try connecting the 6.3 V center tap to the junction of the 6V6 cathodes and the Cathode resistor + cap?

[BobL]

Do you have the DC heaters implemented? If still running AC heaters, did you try connecting the 6.3 V center tap to the junction of the 6V6 cathodes and the Cathode resistor + cap?

I still have AC heaters, but I did move my 6.3v CT to that junction.

[mikeywoll]

Hrm... well, made the following changes to mine tonight:

Added a 4th node, and switched to 22/47/22/22 with 4.7K between nodes B and C and C and D.

Sent node D to the preamp.

Sent grounds for nodes C and D to the same ground as the preamp.

Swapped V2 pins 2/7.

Replaced the 1K resistor on the reverb return grid with 2.2K.

Swapped the A/B switch to the middle lug.

Seems to be humming just as much as it ever did. It's late, so haven't really had a chance to play it or mess with the reverb. Flipping the voicing switch introduces a pretty good pop going from A to B, and does seem to change the tone more. PI balance pot seems to have a different impact on the nature of the hum now... introduces a hiss vs changing the tone of the hum, but I can get rid of the hiss at certain settings. Hum seems here to stay.

Made so many changes, it ain't particularly pretty in there at this point, but pretty confident it is all wired as per my layout.

Ground for the trim pot? That should be pre-amp as well.

Grid resisters on the tube? Hiss sounds like RFI.

Mike

[BobL]

Ground for the trim pot? That should be pre-amp as well.

Grid resisters on the tube? Hiss sounds like RFI.

Mike

I'll start with the trim pot ground.

[mikeywoll]

Looks like the original amp has a CTS balance pot, and the slot looks to be pointing right at the CTS etching (or the outside lug depending on your world view I guess ). That is pretty much where I landed via the dmm method at 3.9K.

Screen Shot 2021-04-21 at 9.02.24 AM.png

Mike

[BobL]

Moving the ground for the balance pot waaaay down to the end w/ the preamp ground does seem to have gotten rid of the bad hum... but also introduced a far more irritating oscillating click and static...

I have the power amp grounded to the same lug as the CT for the PT primary - is this a bad idea?

Using a chopstick to move the input wire around has no impact on this noise - changing the balance pot value impacts overall noise, but doesn't remove it.

Here is the noise: http://www.boblefevremusic.com/temp/mon ... lation.m4a

[dorrisant]

Never had to test an OT for impedance
Not sure I do now since I have the spec sheet right in front of me!?
Care to enlighten me?

Ok... Since you have the spec sheet, this little explanation should be quite helpful. Testing is not necessary. This indicates that you need to change the impedance tap on your OT. Connect your 4Ω tap to your jack and run your 8Ω speaker when you run a pair of 6V6s in place of a pair of EL34s. If you don't you are on the clock, waiting for failure to arrive.

general rule of thumb/ known to work values:

pr 6V6--8K
qd 6V6--4K
pr 6L6--4K
qd 6L6--2K
pr EL84--8K
qd EL84--4K
pr EL34--3.5K
qd EL34--1.75K

There are a couple of books that are of great help with matching tubes to transformers:

The RCA recieving tube manual which is available through Antique Electronic Supply as well as many other sources,
and the GE essential Characteristics manual which is avaiable at the same place.

There is a huge range at which tubes will actually operate, it all depends on what you are trying to make them do, for example, a Trainwreck Express runs a pair of EL34's at a 6.6K primary, and makes about 35 watts class AB with about 400 volts on the plates, but an Aluminum pannel Marshall JMP makes over 50 watts out of a pair of EL34's with a 3.5K primary and about 475 Volts on the plates.


I had an ODS that I built and used a Marshall PT, so It made about 495 plate volts. I stuck a quad of JJ 6V6's in it and hooked it up to my scope and dummy load just to see what I got:

using a 1Khz Test frequency and a Mercury DO100 (2.2K:4,8,16)

I hooked it up to an 8ohm load with the impedance selector on the 8 ohm setting. It put out a respectable 38 watts before the onset of clipping. When I changed the impedance selector to 4 ohms and left the dummy load at 8 ohms (Effectively doubling the primary impedance to 4.4K) the amp put out an amazing 52 watts before the onset of clipping.

The same amp put out 95 watts clean with a quad of Svetlana 6L6's, but made 110 watts with a quad of JJ E34L's.

So you see, there are no set "Rules" only guidelines and records of what has worked previously.

Edited for my own dunbassery!

[mikeywoll]

Moving the ground for the balance pot waaaay down to the end w/ the preamp ground does seem to have gotten rid of the bad hum... but also introduced a far more irritating oscillating click and static...

I have the power amp grounded to the same lug as the CT for the PT primary - is this a bad idea?

Using a chopstick to move the input wire around has no impact on this noise - changing the balance pot value impacts overall noise, but doesn't remove it.

Here is the noise: http://www.boblefevremusic.com/temp/mon ... lation.m4a

Getting somewhere!

That is pretty much where my trim pot ended up grounded as well. The original amp grounds it at the PI ground, FWIW.

Everything power-amp related can safely be ground at the one lug with the PT center tap.

That sounds like RFI, move your grid resistors onto the tubes and maybe check your reverb ground? The reverb is very sensitive to noise. If that doesn't do it you can work out where the noise is coming from by pulling tubes and figuring out which tube is amplifying the noise and working back through the circuit. You can also try adding 10K grid stop resistors on the reverb tube.

resistor -> wire -> tube grid pin = antenna for RFI

wire -> resistor -> tube grid pin = high impedance at the tube = no antenna

I also routinely place 0.1 uf ceramic caps between my input jack ground and chassis (with very short leads) to bleed RF off the guitar cable shield and put 50 pf ceramic caps from plate to cathode on the input tube and reverb tube to bleed off any RF that creeps in otherwise. This is probably not necessary unless you live under a radio tower though .

Mike

[T Wilcox]

Thanks Dorrisant!, I have not seen that before. I have seen the following method but never had a chance to use it.

copied and pasted from the Hoffman forum

How to calculate the output transformer impedance of an OPT with unknown values

Working with tube amps you may need to calculate the output transformer impedance especially when we have an old transformer or a transformer with unknown values. All we have to do is to calculate the turns ratio of the transformer. To do this we need an AC voltage source and an AC voltmeter. So we apply an AC voltage to the primary of the transformer and we measure the output voltage to the secondary.

Turns Ratio = V in (to the primary) / V out ( measured in the secondary) e.g. if we apply 5 Volts AC and we measure 0.21 Volts AC to the secondary the Turns Ratio of the transformer is :
5 / 0.21 = 23.8 That means a turns ratio 24:1

Knowing the Turns Ratio we can calculate the impedance ratio of the unknown transformer and the impedance in a given load to the secondary. The impedance Ratio is the square of the turns ratio:24 X 24 = 576 that means an impedance ratio 576:1

So in a given load impedance of 8 ohms the transformer impedance is 8 X 576 = 4608 ohm ( it is about 4.5K)

I also have never actually calculated Power/watts using my scope so looks like I have some fun experimenting to do with ALL my amps now

Also, I am now curious what I should consider possibly failing when using 6k6 vs 8k primary since it really is not that far off spec considering? Is it the OT or the tubes that are going to have shorter lives?

[mikeywoll]

Thanks Dorrisant!, I have not seen that before. I have seen the following method but never had a chance to use it.

copied and pasted from the Hoffman forum

How to calculate the output transformer impedance of an OPT with unknown values

Working with tube amps you may need to calculate the output transformer impedance especially when we have an old transformer or a transformer with unknown values. All we have to do is to calculate the turns ratio of the transformer. To do this we need an AC voltage source and an AC voltmeter. So we apply an AC voltage to the primary of the transformer and we measure the output voltage to the secondary.

Turns Ratio = V in (to the primary) / V out ( measured in the secondary) e.g. if we apply 5 Volts AC and we measure 0.21 Volts AC to the secondary the Turns Ratio of the transformer is :
5 / 0.21 = 23.8 That means a turns ratio 24:1

Knowing the Turns Ratio we can calculate the impedance ratio of the unknown transformer and the impedance in a given load to the secondary. The impedance Ratio is the square of the turns ratio:24 X 24 = 576 that means an impedance ratio 576:1

So in a given load impedance of 8 ohms the transformer impedance is 8 X 576 = 4608 ohm ( it is about 4.5K)

I also have never actually calculated Power/watts using my scope so looks like I have some fun experimenting to do with ALL my amps now

Also, I am now curious what I should consider possibly failing when using 6k6 vs 8k primary since it really is not that far off spec considering? Is it the OT or the tubes that are going to have shorter lives?

Mismatched OT vs tubes is a big part of the magic of some classic amps. Matchless comes to mind. It's part of the reason they have a harmonically more complex overdrive vs a VOX.

Mike

[pullshocks]

Moving the ground for the balance pot waaaay down to the end w/ the preamp ground does seem to have gotten rid of the bad hum... but also introduced a far more irritating oscillating click and static...

I have the power amp grounded to the same lug as the CT for the PT primary - is this a bad idea?

Using a chopstick to move the input wire around has no impact on this noise - changing the balance pot value impacts overall noise, but doesn't remove it.

Here is the noise: http://www.boblefevremusic.com/temp/mon ... lation.m4a

Hey Bob, I just watched "Raven Lee" on your web site. Great stuff!!!

That new noise sounds awful. I've pretty much lost track of your layout and ground system, maybe post a current gut shot? Did you bring the PI balance pot ground to the negative terminal of the Node C filter cap? If not try that.

[mikeywoll]

Ok... Since you have the spec sheet, this little explanation should be quite helpful. Testing is not necessary. This indicates that you need to change the impedance tap on your OT. Connect your 16Ω tap to your jack and run your 8Ω speaker when you run a pair of 6V6s in place of a pair of EL34s. If you don't you are on the clock, waiting for failure to arrive.

I don't follow... If his transformer reflects 6k6 when connected with an 8ohm speaker on the 8 ohm tap, then all is good. Putting an 8 ohm speaker on a 16 ohm jack will reflect 3k3 ohms to the tubes, right? That doesn't sound like the correct option?

Mike

[BobL]

Moving the ground for the balance pot waaaay down to the end w/ the preamp ground does seem to have gotten rid of the bad hum... but also introduced a far more irritating oscillating click and static...

I have the power amp grounded to the same lug as the CT for the PT primary - is this a bad idea?

Using a chopstick to move the input wire around has no impact on this noise - changing the balance pot value impacts overall noise, but doesn't remove it.

Here is the noise: http://www.boblefevremusic.com/temp/mon ... lation.m4a

Hey Bob, I just watched "Raven Lee" on your web site. Great stuff!!!

That new noise sounds awful. I've pretty much lost track of your layout and ground system, maybe post a current gut shot? Did you bring the PI balance pot ground to the negative terminal of the Node C filter cap? If not try that.

Oh, hey, thanks! We got a whole record of that stuff out there. Bass player in that video died about a month ago, so it's been rough going lately.

I moved the 68K resistor on the input to the tube lug... seems to have changed some things but not improved things...

The balance pot ground I took all the way down to the preamp lug, but I can try it on node C as well.

What is very interesting is that the buzz (sounds like 120 to me now) is *very* dependent on if the amp is upright or not. I leaned it forward to chopstick some stuff and it reduces greatly (and yeah, I followed with my ear to make sure it wasn't just that I couldn't hear it as well ).

It also seems to be very influenced by the reverb circuit - if the tank is plugged in, the oscillation is mostly gone, but the buzz is *bad* now.

If I move the wire going from the reverb pot to the volume pot, it significantly impacts the buzz. If I turn up the reverb knob, it significantly impacts the buzz. If I remove the reverb tube, the oscillation starts to come back, especially with the amp tilted at about 30 degrees...

I had a similar issue in the first amp I built, a JTM45, and it turned out to be a bad capacitor that only was bad when the amp was upright (it was a pain to chase down!)

I had a similar oscillation pop up a while back when I was trying different caps in the reverb circuit, so now I'm thinking there is a very strong chance that the issue is with the .1uF cap I put in there. I'm going to replace that cap and see what happens.